This is what you are most likely to get if you succeed in
contacting the server, but it does not want to talk to you. As the
message suggests, the server refused the connection request
because it found no authorizing entry in its pg_hba.conf
configuration file.

Password authentication failed for user 'joeblow'

Messages like this indicate that you contacted the server, and
it is willing to talk to you, but not until you pass the
authorization method specified in the
pg_hba.conf file. Check the password you are
providing, or check your Kerberos or ident software if the
complaint mentions one of those authentication types.

FATAL 1: user "joeblow" does not exist

The indicated user name was not found.

FATAL 1: Database "testdb" does not exist in the system catalog.

The database you are trying to connect to does not exist. Note that
if you do not specify a database name, it defaults to the database
user name, which may or may not be the right thing.

Note that the server log may contain more information
about an authentication failure than is reported to the client.
If you are confused about the reason for a failure, check the log.